1. Field of the Invention
This invention relates to a comparator circuit, and more particularly to a comparator circuit with a hysteresis that requires relatively less strict accuracy.
2. Description of the Related Art
In a case where a comparator is installed in the electric circuitry in a machine or a system designed to achieve a specific function, the comparator can be forced to receive a signal on which noise has been superposed due to external factors or the intervening circuitry it has passed through. To prevent the comparator from responding to such unwanted noise, the comparator is constructed so that the output V.sub.out may have a hysteresis (a dead band) with respect to the input V.sub.in. With this construction, as the input V.sub.in increases, the output V.sub.out starts to rise at the ON level of the input and remains stable as long as there is no noise large enough to cause the input to decrease to the OFF level. Although not shown, for the decreasing input V.sub.in, the output V.sub.out begins to fall at the OFF level and remains stable insofar as there is no noise large enough to cause the input to increase to the ON level.
Conventionally, such a comparator with a hysteresis comprises a differential transistor pair and a constant current source circuit connected to both the current output terminals of the differential transistor pair. The control terminal of one transistor of the pair receives the signal input, whereas the control terminal of the other transistor of the pair receives the input for a reference voltage. This reference voltage is adjusted by an additional circuit that changes the voltage according to the output signal voltage from the differential transistor pair, with the result that the threshold value for each of the rising and falling of the output signal varies accordingly.
Such a comparator, however, has the disadvantage of requiring so many elements that it is less suitable for an integrated circuit, although it has a hysteresis requiring less strict accuracy of the threshold value. That is, a comparator with the above arrangement needs many of elements to constitute the differential transistor pair, the constant current source circuit, and associated circuits, which results in an increase in the number of elements used in the reference-voltage adjusting circuit. This leads to a rise in production costs and the difficulty in providing a compact design.